Multi-Annual Monitoring of the Water Vapor Vertical Distribution on Mars by SPICAM on Mars Express

Abstract

The distribution of water vapor with altitude has long remained a missing piece of the observational data set of water vapor on Mars. In this study, we present the first multi-annual survey of water vapor profile covering the altitude range from 0 to 100 km based on the SPICAM/Mars Express occultation measurements. During the aphelion season, water remains confined below 40-60 km for all Martian years observed. The highest altitude where water vapor can be spotted is between 70 and 90 km during the southern summer (Ls = 240-300°; perihelion season), approaching the transition between the middle and upper atmosphere. In this season, years without a global dust storm (GDS) show a significant moistening of the upper atmosphere (∼100 ppmv) in the southern hemisphere, confirming a seasonal impact on the hydrogen escape rate. The two observed GDS, in MY28 and MY34, show a substantial disparity in water vapor response. The storm in MY28, which coincides with the southern summer solstice, creates the largest excess of water in both hemispheres at >80 km. This climatology of water vapor will supply a robust statistical basis to address the long-term escape processes of water from Mars.